Powder coating is a process of coating a surface of an object in which a powdered material is applied to the object using electrostatic or compressed air method(s). The object, including the applied powdered material is heated (cured) to the melting point of the powdered material during which it (the powdered material) generally flows to form a smooth film which subsequently dries (cools) to a firm, durable finish exhibiting strong resistance to scratches, cracking, peeling, UV rays, certain chemicals, and rust.
Very generally, the entire powder coating process involves several steps namely: 1) blasting, stripping and/or solvent cleaning an object to remove oils, dirt, rust, etc.; 2) pre-treating the object by exposing it to pre-treatment or conversion chemicals such as phosphate or zinc-based, to further protect it and improve powder adhesion; 3) cleaning and drying the object; 4) applying the powder coat powder to the object, usually with an electrostatic gun or fluidized bed; and 5) curing the applied powder in an oven.
Although finish represents only one attribute of a coated object, it is the most visible and a particularly influential characteristic. Advantageously, powder coatings may have a positive impact on a consumer, both directly and indirectly.
More specifically, powder coatings generally provide superior consistency and uniformity of finish without sags, drips, runs or bubbles. They provide extremely tough, durable films, enhancing high quality products. In general, the performance properties such as impact resistance, flexibility and corrosion resistance of powder coatings are better than those experienced with liquid paints. Powder coated objects resist cracking, peeling and marring during handling and normal use.
In addition, powder coatings are frequently used as a cost saving alternative to liquid paints with respect to energy savings, labor costs, rework costs, material usage, waste disposal and overall efficiency. The advantage to manufactures and consumers is a coated object having a superior quality finish at reasonable cost.
Lastly, environmental issues are of significant interest and importance. Unlike many liquid paints, powder coatings are compliant with environmental regulations. More particularly, liquid paints have traditionally contained solvents that may contribute to air pollution and, in some cases, ozone depletion. Advantageously, powder coatings are free of such pollutants, and properly formulated powder coatings generate no such hazardous waste.
Powders used in powder coating applications generally comprise resins, pigments and additives—while advantageously containing no solvents. And while powder coating powders are available in a wide range of colors and exhibit a broad range of chemical properties, there are two basic types of powders that are widely used in powder coating namely, organic thermoset powders and organic thermoplastic powders.
Organic thermoset powders melt, flow and chemically crosslink within themselves when heated (cured) during a powder coating process to produce a heat stable material that will typically not soften when further exposed to normal heat conditions. In contrast, organic thermoplastic powders do not chemically crosslink when heated (cured) during a powder coating process and thus may be remelted by applying heat. Regardless of the specific type of powder used, there are generally two primary methods of application, namely electrostatic spray and fluidized bed.
In an electrostatic spray process, spray guns are used to apply the powder to an object to be powder coated. Since the powder is electrostatically charged, it will electrostatically adhere to the object.
In a fluidized bed process, dry air is forced through a porous membrane into an open top tank which is about half filled with powder. The air suspends the powder, making it act like a fluid. The object to be coated is preheated above a fusion point of the powder, and then dipped in the fluidized powder, which adheres to the object. An electrostatic fluidized bed is similar to the fluidized bed, except that a high voltage is applied to the powder. When a grounded object is placed into the electrostatic fluidized bed, the powder electrostatically adheres to the object.
Given its commercial importance, methods and/or techniques that improve the finish characteristics of a powder coated object would represent a significant advance in the art. Such a method is the subject of the present invention.